This invention relates in general to hydraulic brake systems and in particular to anti-lock brake systems (ABS) with integral traction control (TC).
Various anti-lock brake systems for motor vehicles have been proposed to prevent wheel lock-up during brake actuation. Wheel rotational behavior is monitored and the corresponding wheel brakes are selectively applied and relieved in order to maintain the wheel speed within a selected slip range while achieving maximum braking forces. Transducers measure wheel rotational behavior and vehicle velocity and supply this information to control electronics. The control electronics control pressure regulating devices which selectively control the pressure at the wheel brake cylinders for proper brake actuation in response to the prevalent braking conditions. Typically, when activated, these systems operate in three pressure control modes, pressure apply, pressure dump and pressure hold. The pressure regulating devices provide the wheel brake cylinders with a pressure increase during the apply mode, a pressure decrease during the dump mode and maintain a constant pressure during the hold mode.
Generally, prior art systems use 2-way/2-position valves as pressure regulating devices for simplicity and cost reduction. A typical circuit configuration for each wheel brake cylinder consists of 1 inlet valve located between a pressurized fluid source and the wheel brake cylinder for pressure application and 1 dump valve located in a return conduit between the wheel brake cylinder and the inlet port of a pressurized fluid source for relieving pressure at the wheel brake cylinder. This results in eight 2-way/2-position valves per anti-lock system.
Traction control of two driven wheels can be added to these systems by including only two additional valves. Traction control reduces wheel spin when the vehicle is accelerating by selectively actuating the brakes of the driven wheels without driver intervention. Control electronics control the same inlet and dump valves used in the anti-lock system to regulate the brakes of the driven wheels during similar apply, dump and hold modes.
Prior art anti-lock systems have used a single slaved dump valve in an attempt to reduce the number of valves required to perform proper pressure regulation. These systems connect the return conduits of two wheel brake cylinders together and use a single 2-way/2-position solenoid valve to relieve the pressure at both wheel brake cylinders. Typically these systems use check valves to prevent pressurized fluid from flowing from one wheel brake cylinder to the other. Such systems have reduced the valve count but sacrifice independent control of the fluid pressure at the two wheel brake cylinders connected to the single dump valve.